1. Field of the Invention
The invention relates to arrangements for synchronizing transmission time of data streams of high-frequency transmitters of a common-wave network.
2. Discussion of the Background
For the USA and other countries such as Canada, South Korea or Brazil, the transmission of digital video and audio data as well as other digital data via terrestrial high-frequency transmitters is regulated by the ATSC (Advanced Television Systems Committee, Washington) standard (e.g., ATSC standard A/53). In practice, this standard has so far been used almost exclusively with reference to digital television transmitters each transmitting at a different frequency. In recent years, the demand for an increasing number of relatively-closely adjacent terrestrial television transmitters has led, in the USA also, to the introduction of so-called common-wave networks for data transmission, within which several transmitters transmit the same information within the same network at the same time and at the same transmission frequency. In addition to an accurate frequency synchronization, this also requires accurate time synchronization, so that the digital data stream is transmitted in exact synchronization by all transmitters in the network.
Various proposals for achieving this time synchronization within the framework of the ATSC standard are already in existence. These proposals share the feature that they do not supply the digital data stream to be transmitted as a continuous data stream to the transmitters; instead, the digital data stream to be transmitted is subdivided in a master station into a periodic succession of data frames, which are, in fact, controlled by a time reference, for example, derived from the GPS (Global Position System) (e.g., 1 pps pulse together with the reference frequency of 10 MHz transmitted via GPS). Within these data frames, so-called synchronizing time stamps are inserted at given positions of the data packets. The data frames prepared in this manner are then transmitted from the master station to the individual high-frequency transmitters via an appropriate distribution network (e.g., cable, radio channel, satellite channel, etc). In the individual transmitters, these data frames are supplied to a signal-processing unit with a delay element, where they are coded and modulated according to the ATSC standard.
Moreover, the setpoint transmission time determined according to the standard, at which the data frames are to be transmitted at the output of the individual transmitters, is calculated within the transmitters, in fact, from the synchronizing time stamps inserted in the data frames with reference to the same time reference, which was also used in the master station, that is to say, for example, once again, with reference to the 1 pps pulse of the GPS system. The setpoint transmission time calculated in this manner determines the necessary delay of the data frames within the delay device. The data frames delayed in the delay device are finally transmitted with a system clock supplied by a freely-oscillating clock generator provided in the transmitter, which is synchronized, for example, once again, via the 10 MHz reference frequency of the GPS, to the output of the transmitter, from which they are finally transmitted via the antenna.
Initial experiments with these known proposals for expanding the ATSC standard for common-wave networks have shown that short-term interruptions of the transmitted signal frequently occur. The known proposals actually only function in a stable manner if all of the signals are exactly synchronized with one another, that is to say, the data rate and frame lengths of the data signal; the time reference (e.g., 1 pps of the GPS receiver); and the system clock of the clock generator generated in the transmitter itself, with which the transmitted signal is transmitted. In practice, however, this is often not the case, because many GPS receivers do not actually deliver the 1 pps pulse and the reference frequency 10 MHz in a synchronous manner. Accordingly, the “curved-value” system clock provided for reading out the data frames from the delay device cannot always be generated with 100% accuracy. Moreover, the supply routes from the master station to the transmitters can change the data rates and therefore the frame periods of the data signal, for example, in the case of satellite supply and similar, as a result of Doppler effects. In consequence, the delay time calculated in the transmitter and set in the delay element is no longer correct, but deviates to a greater or lesser extent from the setpoint value. According to the known systems, a new delay time is calculated and set in such cases. However, this leads to an interruption of the transmitted signal with the associated disturbances, as already mentioned.